Many vacuum fluorescent electronic instrument cluster systems consists of the following two sections:
1) the information processing and controlling section (host microcomputer); and PA1 2) the VF display driver section (display driver microcomputer). PA1 Anode data for the next grid is shifted into the input shift register section of the latch driver while the data in the output latch section is being applied to the anodes. When the current grid period expires, ALL grids are turned off for a period of time (inter-grid blank time, IGB) and then the anode data from the input shift register is transferred to the output latch and the next grid is turned on. While this grid is "on", anode data for the next grid is shifted into the input shift register and the entire process repeats itself.
The host microcomputer gathers and processes information and communicates that information to the display driver. The display driver handles the interface to the VF display.
A VF display consists of a filament (hot cathode), grids, and anodes. A display segment appears lit when electrons emitted from the filament, pass through its associated grid and strike its corresponding anode causing the phosphor on the anode to glow.
The filament is a thin wire that, when heated by a current, provides a source of electrons. This current is AC on large displays to ensure even brightness across the display. The anodes, by being more positively charged than the filament, attract the electrons necessary to make the phosphor glow. The grid is between the filament and the anode and is used to regulate the flow of electrons from the filament to the anode. The grid controls electron flow by controlling the field between the cathode and the anode such that either many or no electrons leave the filament and continue on to the anode. In order for a display segment to appear lit BOTH the anode and the grid for that anode must be on. If either the grid or the anode is off the segment will be off.
A latch driver for providing anode data may comprise a two section device consisting of an input shift register and an output latch. Data that is in the output latch section is independent of data in the input shift register section. Control signals allow data to be transferred from the input shift register to the output latch. Any data in the output latch is applied to the anodes of the VF tube. Each output of the latch may be connected to many VF tube segments. The segment that is currently being addressed depends on which grid is on.
The VF tubes in an electronic instrument cluster are often designed to operate under a 4:1 multiplex scheme (i.e. four VF tube segments). This means that a multiplex period (TM) is broken into 4 parts called grid periods (TG). A complete set of data is sent to the VF tube each multiplex period with 25% of the data being sent during each of the 4 grid periods. Anode data that is under control of a specific grid is active (on or off depending on the actual anode data) during that grid's ON time.
In order to produce a display, the following sequence typically takes place:
Once all grids are off, they must stay off for the inter-grid blank time, IGB. The IGB time is required to avoid having more than one grid on at a time. Since the grid voltage cannot be turned off instantaneously, there is a fall time associated with it. The IGB time must be long enough to encompass the fall time to insure that the previous grid is completely off before the next grid is turned on. When the IGB time has expired the next grid is turned on.
Display brightness is related to the potential difference (DC voltage) between the filament and the anode, and the grid on-time. The larger the potential difference and the longer the grid is left on, the brighter the display.
To obtain a maximum brightness display each grid will be left on for the maximum time possible which is the grid period minus the delays. Therefore, the grid on-time (TGON)=(TG-IGB). The longer IGB is, the shorter the maximum achievable grid on-time becomes.
Display brightness is varied by varying TGON. As TGON becomes shorter the display brightness becomes dimmer. (The shortest grid on-time achievable depends on the speed of the microcomputer and any propagation delays in the circuitry.)
A changing or flickering brightness problem develops when the grid on-time becomes smaller than the period of the AC filament signal (TGON&lt;TF). The grid will be on only during a portion of the filament signal, and since the filament signal is asynchronous to the grid signal, the display brightness will fluctuate.
Most conventional display systems cannot use TGON values that are smaller than TF without flickering and this limits their ability to produce a very dim display which is continuously variable down to the point of barely discernable.
U.S. Pat. No. 4,859,912 discloses a brightness control circuit which overcomes part of this problem. A feedback signal from the power supply that generates the AC filament signal is used as an input to a microcomputer. The microcomputer uses this signal to synchronize turning on the anode with the filament signal.
U.S. Pat. No. 4,158,794 discloses a VF display control system which maintains substantially constant illumination across the display by controlling power to the cathode filament in response to driven and undriven states of the control grids.
U.S. Pat. No. 4,495,445 discloses a VF display control system which produces uniform brightness by applying a control signal which is in phase with the AC voltage applied to the cathode/filament of the display.
U.S. Pat. No. 4,719,389 discloses a VF display control system which uses a microcomputer to synchronize filament voltage with grid voltage to maintain a flicker-free display.
Other U.S. patents which disclose VF display control systems generally of the type to which this invention relates include U.S. Pat. Nos. 4,791,337 and 4,388,558.